For treating gases, and in particular for purifying the polluting exhaust gases of combustion engines, there have been used various catalytic surfaces, mainly depending on the composition of the exhaust gases and the structure of the catalyst. The polluting components of exhaust gases are substantially composed of unburnt hydrocarbons, cracked hydrocarbons, carbon monoxide, and nitrogen oxides. The purpose of the catalyst is to cause the hydrocarbons and the carbon monoxide to oxidize, i.e. to burn as completely as possible to carbon dioxide and water, and the oxides of nitrogen to become reduced to elemental nitrogen. Exhaust gas often also contains small amounts of sulfur dioxide derived from the sulfur present in the fuel, the sulfur burning in the engine to sulfur dioxide.
The basic structure of a catalyst comprises a frame material, i.e. a carrier, which may be ceramic or a metal foil strip; a support layer, i.e. a so-called washcoat; and noble metals used as active agents. A ceramic frame is usually made up of one or two successively placed monolithic pieces or honeycombs, which have channels or conduits, separated from each other by thin walls and longitudinal relative to the gas flow direction. A honeycomb made of a metal foil strip is made either by winding or by stacking, in which case, after tight packing of the strips, the corrugations transverse to the strip, in at least in a portion of the strip, form channels or conduits in the gas flow direction. Typically a honeycomb is made up of a flat strip and a corrugated strip placed alternately.
When the purification of the exhaust gases of a gasoline engine is concerned, gamma aluminum oxide is commonly used as the support. In catalysts used for purifying exhaust gases of other types of engines, for example diesel engines and natural gas engines, it is also possible to use, for example, silica, i.e. silicon dioxide, and titania, i.e. titanium dioxide. Zeolites are also commonly used as supports in catalysts. The purpose of the support is to form a sufficiently large surface area for the catalytic agents such as noble metals, and often also to participate in the catalytic reactions. Depending on the support, such support properties having an effect on reactions may include an ability to bind sulfur dioxide gas or oxygen. The physical structure of the catalytic surface means, for example, the thickness of the support, the layered structure of the support, the porosity of the support, and other properties which are dependent on the coating technique used.
Various additives, such as earth alkali metals and lanthanides, are used for improving the properties of the support, for example its thermal stability. Cerium compounds are used in the support to improve the storage of oxygen. In application publication FI-884977, the cerium oxide content of a support is varied according to the form in which aluminum oxide is used as the support.
Application publication FI-884978 discloses a catalyst in which the catalytic surface contains as a support cerium oxide, platinum and palladium in addition to aluminum oxide. Application publication JP 63,162,045 discloses a catalyst which contains noble metals such as Pt, Pd and Rh in various concentrations and in the form of various salts. A flat metal foil and a corrugated metal foil are immersed in a slurry of Al.sub.2 O.sub.3 to produce a washcoat layer. The metal foil strips are wound to form a honeycomb, one end of which is immersed in a Pt ammine solution and the other end into a PdCl.sub.2 solution. Finally the entire catalyst is immersed in a solution of RhCl.sub.3. Thus a honeycomb is obtained which has catalytic surfaces containing different noble metals in different parts of the honeycomb.
U.S. Pat. No. 4,382,323 discloses a metal catalyst in which different noble metals have been provided on different surfaces of an individual channel. The support on all surfaces is gamma aluminum oxide which has been stabilized with barium and to which cerium has been added to even out the need for oxygen during catalytic reactions.
It is also previously known to make two separate shorter honeycombs the supports and/or noble metals contained in them differing from each other. By placing the honeycombs in succession or within a common casing, a catalyst is obtained which has catalytic surfaces containing different noble metals in different parts of the catalyst.
The catalysts in accordance with the state of the art described above have the drawback that the support-dependent behavior of the noble metals has not been taken into account in them. It has namely been observed, surprisingly, that certain noble metals work optimally only in connection with certain supports. This surprising interaction promoting the catalytic action of a noble metal will be shown more clearly in the accompanying examples.